Polymers and copolymers of hydroxy acids, generally known as poly(hydroxy acid)s, slowly hydrolyze and biodegrade to environmentally benign products. These materials are well behaved thermoplastics with appealing aesthetic qualities. Consequently, high molecular weight poly(hydroxy acid)s (molecular weight of at least 10,000 and normally 15,000 to 500,000), particularly polymers and copolymers of lactic acid and glycolic acid, are potential replacements for poly(styrene) and other non-biodegradable polymers in numerous applications, especially packaging.
Poly(hydroxy acid)s degrade extensively during processing at 180 C. and above, especially when the polymer has not been thoroughly dried and maintained in a dry environment during processing. Both weight loss and molecular weight reduction occur. Poly(lactic acid), for example, undergoes a dramatic decrease (50-88%) in molecular weight during injection molding at 130-215 C. (S. Gogolewski, et al, Polymer Degradation and Stability, 40, 313-22, 1993).
Hydrolysis, depolymerization and cyclic oligomerization, and intermolecular and intramolecular transesterification were observed when poly(lactic acid) was heated above 190 C. (K. Jamshidi, et al, Polymer, 29, 2229-2234, 1988). Trace amounts of stannous octoate polymerization catalyst were shown to have a strong effect on thermal degradation. The melting temperature of crystalline poly(L-lactic acid) was reported to approach 184 C. as molecular weight increased, indicating that poly(L-lactic acid) must generally be thermally processed near the temperature at which it thermally degrades.
The limited thermal processibility of poly(hydroxy acid)s, especially poly(lactic acid) and its copolymers, severely limits their applications, particularly as replacements for non-biodegradable polymers. A need exists for poly(hydroxy acid)s that with improved stability to thermal processing conditions.